Newer generations of semiconductor devices use ever decreasing design rules to form a semiconductor device. Current technologies require minimal spacing between components while keeping the components electrically isolated from one another. In many technologies, field isolation regions are used to electrically isolate components from one another. The field isolation regions may be formed by many different methods. One of the most common field isolation methods forms field isolation regions by local oxidation of silicon (LOCOS).
A conventional LOCOS field isolation process uses a pad oxide layer and a silicon nitride layer, wherein the pad oxide layer has a thickness of about 40 percent of the thickness of the silicon nitride layer. The 40 percent thickness is large enough to minimize defects, yet small enough to minimize encroachment. The defects may include edge dislocations, that form in an underlying substrate along the edges of the field isolation regions. Still, the conventional LOCOS field isolation process allows too much encroachment into an active region defined by the field isolation regions. For example, if the pad oxide layer has a thickness of about 400 angstroms and a silicon nitride layer of about 1000 angstroms, the amount of encroachment at the edge of each field isolation region is about one angstrom for every angstrom of field oxide grown. If the field isolation regions are grown to a thickness of about 6000 angstroms using the previously given pad oxide and silicon nitride layer thicknesses, the encroachment is about 6000 angstroms or 0.6 micron. The areas where encroachment occurs are generally wasted areas because those areas are not generally part of the active regions and act as a poor quality field isolation region. The conventional LOCOS field isolation process is commonly used for semiconductor devices designed with design rules of 1.2 microns and higher.
The encroachment may be reduced by thinning the pad oxide layer and/or thickening the silicon nitride layer. For example, some technologies may use a pad oxide layer of about 200 angstroms and a silicon nitride layer of about 1800 angstroms. Encroachment for a 6000 angstrom field isolation region with the thinner pad oxide layer and the thicker silicon nitride layer may be 0.3 micron. During the formation of the field isolation regions, tremendous stress may be exerted on the underlying substrate near the edges of the field isolation regions. If the stress becomes too great, the stress is usually relieved by generating defects, such as edge dislocations, within the substrate. The defects are believed to be generated because the pad oxide layer is less than 40 percent of the thickness of the silicon nitride layer. Although the encroachment is reduced, too many defects within a substrate may be generated near the edge of the field isolation regions.
Poly-buffered LOCOS (PBL) field isolation process uses a polycrystalline silicon (polysilicon) layer between the pad oxide layer and the silicon nitride layer. During the field isolation region formation, the polysilicon layer is supposed to absorb some of the stress to reduce defect generation within a silicon substrate. Those skilled in the art believe that a PBL field isolation process should be capable of being used with design rules as small as 0.25 micron or possibly lower.